1. Salt resistance in the 6.5% NaCl broth is determined by turbidity.
2. Bile esculin agar is really 2 different tests---resistance in the
presence of bile and the use of the sugar esculin.
a. If bile resistant, you will see growth on the slant.
b. If the bacterium is bile resistant AND it uses esculin, the
slant will turn the color of black coffee.
c. Unfortunately, if your bacterium is NOT resistant to bile, it
will not grow and, obviously, it cannot use esculin then.
d. We have esculin sugar alone that could be used to identify
your bacterium if it does not grow in the presence of bile.
3. For the SXT disc, look for any size zone. The zone around
bacitracin disc should be 14mm or larger, and the optochin zone
should be 16mm or greater.
4. Use the chart below to presumptively identify your Streptococcus
species, if possible to do so.
5. Take a small amount of colony growth from your blood agar plate and place on a clean
glass slide. Add a drop of catalase reagent (H2O2). Organisms that produce catalase
(peroxidase) will cause the reaction H2O2 H2O + O2 (gas) with very visibly bubbling.
6. The Streptococci are catalase negative and do not cause a bubbling reaction. This is
in contrast to the Staphylococci which are catalase positive and produce very strong
bubbling.